1. Field of the Invention
The present invention relates to an aqueous resin composition that can be utilized in a wide variety of fields including paints for vehicles, paints for cans, paints for the outer facings of buildings, adhesives, inks, impregnating agents for fibers and paper, and sealing agents and the like.
This application is the U.S. National Stage of International Application No. PCT/JP03/07209, filed on Jun. 6, 2003, the content of which is incorporated herein by reference.
Priority is claimed on Japanese Patent Application No. 2002-172679, filed Jun. 13, 2002, the content of which is incorporated herein by reference.
2. Description of Related Art
In recent years, environmental protection measures have required reductions in the volatile matter such as solvents that is discharged from paints, and the substitution of solvent based paints with water based paints is occurring in most fields.
For example, in the vehicle paint application field, large quantities of solvent based paints are used, and reducing the quantity of organic solvents discharged from these paints is now a primary concern, and the replacement of solvents based paints with water based paints is being investigated for paints used in all of the paint application processes, including priming, intermediate coats, and top coats.
Of the above types of paint, paints used in the intermediate coating of vehicles require high performance characteristics including resistance to chipping (chipping: paint film damage caused by small stones and the like flying off the road and striking the paint film), good adhesion to paint films such as the priming coating, good storage stability (pigment dispersibility), solvent resistance, and a favorable external appearance, and as a result, resin compositions comprising an organic solvent based polyester resin and an amino resin curing agent as the primary components have been used conventionally.
In recent years, aqueous coatings have started to be used in some vehicle intermediate coating materials, although an aqueous paint for use as a vehicle intermediate coating that is able to provide a high level of performance for all of the above required characteristics is still to be developed.
For example, aqueous intermediate coating materials comprising a polyester resin, a curing agent, and an amine salt of an organic sulfonic acid have been reported (for example, patent reference 1: Japanese Unexamined Patent Application, First Publication No. Hei 11-76937), and the disclosed technique enables a reduction in the quantity of organic solvent discharged during the vehicle paint application process. However, these aqueous intermediate coating materials display inadequate pigment dispersibility, and problems arise during storage, including coagulation or precipitation of the pigment.
Known techniques for resolving problems associated with the storage stability of the paint, arising from factors such as coagulation or precipitation of the pigment, or separation and precipitation of the resin, typically involve improving the pigment dispersibility using a pigment dispersing agent such as a surfactant or the like. However, in order to achieve the desired effect using this technique, a significant quantity of the pigment dispersing agent must be used, and because this pigment dispersing agent remains in the final paint film, it has a deleterious effect on the physical properties of the paint film such as the water resistance and the like, which presents a considerable practical problem.
Furthermore, it has also been reported that a paint produced using a pigment and a water dispersible alkyd resin composition, in which a hydrophobic alkyd resin having an acid value of no more than 5 and a hydroxyl value of 1 to 50, and a hydrophilic alkyd resin having an acid value of 10 to 20 and a hydroxyl value of no more than 5 are bonded together chemically, displays minimal variation in viscosity over time, and offers excellent storage stability (for example, patent reference 2: Japanese Unexamined Patent Application, First Publication No. Hei 5-287184). However, these water dispersible alkyd resin compositions display a structural characteristic wherein the surface of the dispersed particles within the aqueous medium are coated with the hydrophilic alkyd resin having a hydroxyl value of no more than 5, and the hydrophobic alkyd resin having a hydroxyl value of 1 to 50 exists internally, inside the particles, in other words, a structure in which hydroxyl groups are localized inside the particles, and consequently the curability of the composition when an amino resin or the like is used as the curing agent is poor, and the physical characteristics of the paint film, such as the solvent resistance, tend to be inferior.
Furthermore, it has also been reported that an aqueous resin composition comprising an aqueous alkyd resin produced by condensing a specific vinylated fatty acid with a polyol compound, and then performing a neutralization with a basic compound, displays excellent resin stability and paint stability, and also offers excellent overall storage stability (for example, patent reference 3: Japanese Unexamined Patent Application, First Publication No. Hei 9-111184). However, if a water based paint that utilizes this type of aqueous resin composition is used as the intermediate coating material for a vehicle, although the storage stability of the paint is excellent, the chipping resistance of the produced paint film is unsatisfactory.
As described above, substituting conventional solvent based paints with water based paints, while retaining a good balance between the typically required characteristics for a vehicle intermediate coating material such as chipping resistance, and ensuring excellent pigment dispersibility and storage stability is extremely difficult, and at present no such water based paint is available, although the need for a water based paint capable of satisfying the above required characteristics to a high level is growing rapidly stronger in the marketplace.